The efficient catalytic trimerization and tetramerization of olefinic monomers, such as the trimerization and tetramerization of ethylene to 1-hexene and 1-octene, is an area of great interest for the production of olefinic trimers and tetramers of varying degrees of commercial value. In particular, 1-hexene is a valuable comonomer for linear low-density polyethylene (LLDPE) and 1-octene is valuable as a chemical intermediate in the production of plasticizer alcohols, fatty acids, detergent alcohol and lubrication oil additives as well as a valuable comonomer in the production of polymers such as polyethylene. 1-Hexene and 1-octene can be produced by a conventional transition metal oligomerization process, although the trimerization and tetramerization routes are preferred.
Several different catalytic systems have been disclosed in the art for the trimerization of ethylene to 1-hexene. A number of these catalysts are based on chromium.
Chem. Commun., 2002, 8, 858-859 (BP), discloses chromium complexes of ligands of the type Ar2PN(Me)PAr2 (Ar=ortho-methoxy-substituted aryl group) as catalysts for the trimerization of ethylene. U.S. Pat. No. 6,800,702 (BP) discloses a catalyst for the trimerization of olefins comprising a source of chromium, molybdenum or tungsten, a ligand containing at least one phosphorus, arsenic or antimony atom bound to at least one hydrocarbyl or heterohydrocarbyl group having a polar substituent, but excluding the case where all such polar substituents are phosphane, arsane or stibane groups, and optionally an activator. The ligand used in most of the examples is (2-methoxyphenyl)2PN(Me)P(2-methoxyphenyl)2.
Although the catalysts disclosed in the BP documents mentioned above have good selectivity for 1-hexene within the C6 fraction, a relatively high level of by-product formation (e.g. C10 by-products) is observed.
Catalytic systems for the tetramerization of ethylene to 1-octene have recently been developed. A number of these catalysts are based on chromium.
WO 2004/056478 and WO 2004/056479 (Sasol) disclose catalyst compositions and processes for the tetramerization of olefins. The catalyst compositions disclosed in WO 2004/056478 comprise a transition metal and a heteroatomic ligand having the general formula (R)nA—B—C(R)m where A and C are independently selected from a group which comprises phosphorus, arsenic, antimony, oxygen, bismuth, sulphur, selenium, and nitrogen, and B is a linking group between A and C, and R is independently selected from any homo or heterohydrocarbyl group of which at least one R group is substituted with a polar substituent and n and m is determined by the respective valence and oxidation state of A and C. The catalyst compositions disclosed in WO 2004/056479 comprise a transition metal and a heteroatomic ligand having the general formula (R)nA—B—C(R)m where A and C are independently selected from a group which comprises phosphorus, arsenic, antimony, oxygen, bismuth, sulphur, selenium, and nitrogen, and B is a linking group between A and C, and R is independently selected from any homo or heterohydrocarbyl group and n and m is determined by the respective valence and oxidation state of A and/or C.
WO 2004/056480 (Sasol) discloses the tandem tetramerization and polymerisation of ethylene. Specifically, WO 2004/056480 discloses a process for polymerising olefins to produce branched polyolefins in the presence of a distinct polymerization catalyst and a distinct tetramerization catalyst, wherein the tetramerization catalyst produces 1-octene in a selectivity greater than 30% and the 1-octene produced is at least partially incorporated into the polyolefin chain.
Although the tetramerization catalysts disclosed in the Sasol documents mentioned above have good selectivity for 1-octene within the C8 fraction, again, a relatively high level of by-product formation is observed. Typically, the by-product consists of C6 compositions; however, only about 70 to 80% wt. of the C6 by-product composition is 1-hexene, with the remaining C6 by-product comprising compounds such as methylcyclopentane and methylenecyclopentane. The presence of these remaining C6 by-product compositions, which have very little commercial use or value, is highly undesirable from both an economic point of view as well as from a product separation point of view.
It has now been surprisingly found that the catalyst system and process of the present invention provide an efficient route for the selective production of 1-hexene and 1-octene from ethylene while reducing the level of by-product formation, especially C10 by-products, solids (i.e. heavy waxes and/or polyethylene) and C6 compositions/isomers other than 1-hexene.